Instrumented impact tests are currently used to measure the dynamic fracture toughness K.sub.Id of materials. During the impact event, the load at the peen of the striking hammer is measured as a function of time or deflection of the specimen. From the critical load for onset of crack propagation the dynamic fracture toughness value K.sub.Id is derived utilizing the conventional static stress intensity factor formulas (ASTM STP 466 (1970)--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia; ASTM STP 563 (1974)--Instrumented Impact Testing, American Society for Testing and Materials, Philadelphia; IIW (International Institute of Welding, Commission X), U.K. Briefing Group on Dynamic Testing (1976), "Some Proposals for Dynamic Toughness Measurement," Proc. Int. Conf. Dynamic Fracture Toughness, London; PVRC/MPC Joint Task Group on Fracture Toughness Properties for Nuclear Components, Working Group on Instrumented Precracked Charpy Test, Chairman C. Buchalet, Westinghouse Nuclear Energy Systems (1974), "Recommended Procedure for Instrumented Precracked Charpy Testing").
However, difficulties are inherent with this measuring and evaluation procedure: first, because the load time records oscillate and often cause uncertainties in the determination of the actual fracture load, and secondly, because a dynamic material strength value K.sub.Id is inferred from a static evaluation analysis.
The conventional measuring technique therefore can only yield meaningful data if fracture occurs only after time sufficiently long that a quasi static loading condition has been reached in the specimen. For shorter loading times to fracture dynamic effects can strongly influence the stress state in the specimen (Glover, A. P., F. A. Johnson, J. C. Radon and C. E. Turner (1976), "Dynamic Fracture Toughness Measurement by Instrumented Bend Testing and Compact K Testing," Proc. Int. Conf. Dynamic Fracture Toughness; Kalthoff, J. F., S. Winkler, and J. Beinert (1979), "The Influence of Dynamic Effects in Impact Testing," Int. Journ. of Fracture, 13, pp. 528-531; Ireland, D. R. (1976), "Critical Review of Instrumented Impact Testing," Proc. Int. Conf. Dynamic Fracture Toughness, London; Loss, J. F., J. R. Hawthorne, and C. A. Griffis (1975), "Fracture Toughness of Light Water Reactor Pressure Vessel Materials," Naval Research Laboratory Memorandum Report 3036; Radon, J. C., and C. E. Turner (1969), "Fracture Toughness Measurements By Instrumented Impact Test," J. Engng. Frac. Mech., Vol. 1, No. 3, p. 165; Turner, C. E. (1970), "Measurement of Fracture Toughness by Instrumented Impact Test"; ASTM STP 466--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia, pp. 93-114; Venzi, S., A. H. Priest, and J. J. May (1970), "Influence of Inertial Load in Instrumented Impact Tests;" ASTM STP 466--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia, pp. 165-180; Winkler, S., J. F. Kalthoff, and A. Gerscha (1979), "The Response of Pressure Vessel Steel Specimens on Drop Weight Loading," Proc. 5th Int. Conf. on Structural Mechanics in Reactor Technology, Berlin, Vol. G(4/6), North-Holland Publishing Company). If these dynamic influences on the measured load records are not taken into account, erroneous data can be obtained which may lead to an overestimation of the true toughness of the material (IIW (International Institute of Welding, Commission X), U.K. Briefing Group on Dynamic Testing (1976), "Some Proposals for Dynamic Toughness Measurement," Proc. Int. Conf. Dynamic Fracture Toughness, London; Glover, A. P., F. A. Johnson, J. C. Radon, and C. E. Turner (1976), "Dynamic Fracture Toughness Measurement by Instrumented Bend Testing and Compact K Testing," Proc. Int. Conf. Dynamic Fracture Toughness, London; Matthews, W. T. (1970), "The Role of Impact Testing in Characterizing the Toughness of Materials," ASTM STP 466--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia, pp. 3-20; Turner, C. E. (1970), "Measurement of Fracture Toughness by Instrumented Impact Test," ASTM STP 466--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia, pp. 93-114. It is postulated, therefore (Ireland, D. R. (1976)), "Critical Review of Instrumented Impact Testing," Proc. Int. Conf. Dynamic Fracture Toughness, London; PVRC/MPC Joint Task Group on Fracture Toughness Properties for Nuclear Components, Working Group on Instrumented Precracked Charpy Test, Chairman C. Buchalet, Westinghouse Nuclear Energy Systems (1974), "Recommended Procedure for Instrumented Precracked Charpy Testing;" Turner, C. E. (1975), "Dynamic Fracture Toughness Measurements by Instrumented Impact Testing," Advanced Seminar on Fracture Mechanics, Commission of the European Communities, Joint Research Centre, (Ispra, Italy) that the quasi static procedure can be applied only when the time to fracture t.sub.f of the specimen is longer than about three times the period .tau. of the characteristic oscillation of the impacted specimen: EQU t.sub.f &gt;3.tau.
The period .tau. is given approximately by the empirical formula (IIW (International Institute of Welding, Commission X)), U.K., Briefing Group on Dynamic Testing (1976), "Some Proposals for Dynamic Toughness Measurement," Proc. Int. Conf. Dynamic Fracture Toughness, London; Glover, A. P., F. A. Johnson, J. C. Radon, and C. E. Turner (1976), "Dynamic Fracture Toughness Measurement by Instrumented Bend Testing and Compact K Testing," Proc. Int. Conf. Dynamic Fracture Toughness, London; Matthews, W. T. (1970), "The Role of Impact Testing in Characterizing the Toughness of Materials," ASTM STP 466--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia, pp. 3-20; Turner, C. E. (1970), "Measurement of Fracture Toughness by Instrumented Impact Test," ASTM STP 466--Impact Testing of Metals, American Society for Testing and Materials, Philadelphia, pp. 93-114 EQU .tau.=1.68 (S.multidot.W.multidot.B.multidot.C.multidot.E.sup.1/2 /c.sub.1
where S is the support span, W and B are the width and the thickness of the specimen, C is the specimen compliance, E is Young's modulus and c.sub.1 is the longitudinal wave speed. Short periods, therefore, result for small specimen dimensions. Long fracture times t.sub.f, on the other hand, are obtained only when rather ductile materials are tested at low impact velocities. The condition (1) therefore restricts the applicability of impact tests in an unsatisfactory way:
Specimens of large dimensions, which are often required for a valid toughness test, cannot be utilized; PA0 Materials which fail in a more brittle manner cannot be investigated; PA0 The maximum allowable loading rate is limited.